This paper explores the relations between the lexicon and the initial stage of the syntactic component. The main goal is tofind out ifthere is a covariation between the changes in the lexicon (when it reaches 400 words) together with the increase in grammatical words (closed class words), with morpheme inflection and changes in syntax measured in MLUw.The results show a linear increase in grammatical lexical items, but in all subjects there is a brief stage of fast non-monotonous increase when their lexicon comprises 400 words (or slightly before). It is confirmed that the evolution of both components (lexicon and syntax) is not linear, suggesting a reorganization of the initial syntactic component that is related to the size of the lexicon and the non monotonous increase of the closed class words.